russray,
JUnit 3.x support is provided by the base Eclipse JDT, so I think you could probably find a request at http://bugs.eclipse.org for 4.0 support added soon (maybe in the next release after 3.2?)
But in the mean time, dropping 4.0 libraries into Eclipse 3.1 should work for you as it supports Java 5, but I understand that out of the box support with the JUnit integrated tools is probably what you want.
(Maybe 3.2 added Junit 4 support? I haven’t been following it)